Reducing nitrate leaching and runoff through crop rotations in the Upper Mississippi River Basin

Nitrate nitrogen (NO3-N) loss is the main source of water quality pollution in many agricultural regions. Mitigating NO3-N loss from croplands in a tractable manner has become a vital challenge. This study selected a heavy NO3-N loading area of the Upper Mississippi River Basin (UMRB) and applied a calibrated Soil and Water Assessment Tool (SWAT) model. The model was used to evaluate lateral and vertical NO3-N losses before and after crop rotation adjustments. Results showed that the risk of high NO3-N losses was greater under continuous corn than that of corn-soybean rotation in the baseline land uses, and NO3-N leaching was a more severe loss pathway than runoff. Moreover, the four crop rotation adjustment scenarios were effective in reducing NO3-N losses for the UMRB, especially for the lower reaches zones. The appropriate crop rotation patterns were then defined as meeting the 20 % reduction target threshold with the least deviation from baseline land uses. It was found that the appropriate patterns could reduce NO3-N leaching over 20 % compared to the baseline land uses, and also simultaneously reduced NO3-N runoff by more than 25 %, which further supported the appropriateness of selected crop rotation patterns. The final adjustments were mainly concentrated in the headwaters and lower reaches zones of the basin, primarily for continuous corn land use which had more severe risks. Overall, this study provided insights for mitigating NO3-N losses through runoff and leaching and highlighted the importance of integrated basin-wide management from multiple perspectives.